In the case of a centrifugal fan, a gas sucked in through an air inlet is pushed radially outward by a centrifugal force produced by rotation of an impeller, and then whirls in a circumferential direction through an annular air channel portion provided around the impeller, and the gas is thereafter discharged outwardly through an air outlet.
In a medical appliance such as a respirator or a sputum aspirator, a centrifugal fan generally needs to be used to cause a flow of a gas in order to provide an aid in respiration or to facilitate suction. The centrifugal fan used in such a medical appliance is required to have a high static pressure and a large air volume, and, in addition, to produce little noise in view of an environment in which the medical appliance is used.
JP-A 9-14192, for example, describes a centrifugal fan which includes an air current guide at an air inlet, and which thereby achieves reduced noise.
In the centrifugal fan described in JP-A 9-14192, the ratio of the area of an imaginary circle joining center-side ends of blades of a centrifugal impeller to the area of an entry of the air inlet is optimized while the air current guide is provided to smoothly guide an air current at the air inlet. In addition, the air current guide is arranged to be in the shape of a circular arc in a cross-section, and a seal member which is in sliding contact with an entry portion defined at a center of a side plate of the centrifugal impeller is attached to an inside of the circular arc so that the air inlet can be kept airtight. That is, provision of the air current guide and the seal member is necessary to achieve a high static pressure and low noise, and this leads to a complicated structure.